The present invention relates to a gas detector capable of simultaneously detecting the kind and concentration of a gas to be detected, by using as a gas sensitive material a thin film containing organic dye molecules.
Among conventional gas detectors, a contact combustion type or a semiconductor type has been a mainstay. In these gas detectors, a gas sensitive material is formed by making use of an inorganic semiconductor, a ceramic, or a precious metal catalyst. Meanwhile, gas detectors using an organic molecule thin film as the gas sensitive material have been proposed in recent years. It is known that certain types of organic dye molecules interact with an oxidizing or reducing gas with a certain degree of selectivity, and their electrical or optical characteristics change. For instance, an application to NO.sub.x, SO.sub.x gas detectors has been proposed by making use of the change in electrical characteristics of phthalocyanine. Furthermore, an absorption-measuring type gas detector has also been proposed in which a gas sensitive thin film formed of organic molecules is formed on a piezoelectric vibrating element such as a quartz resonator or a surface acoustic wave device (SAW device), and a gas exhibiting a high adsorptivity with respect to the gas sensitive thin film, such as an oxidizing or reducing gas or organic solvent-based gases of various kinds, is detected as a change in the oscillation frequency of the piezoelectric vibrating element.
For instance, B. Holcroft and G. G. Roberts have confirmed that the oscillation frequency of a SAW device decreases by approx. 1 kHz by accumulating a silicon phthalocyanine LB film on a 98.6 MHz delay-line type SAW device and placing the same in a 100 ppm NO.sub.2 gas atmosphere, and that the change in the frequency is proportional to the concentration of the reactive gas. In addition, Moriizumi et al. have made an attempt in which different organic thin films are formed on a quartz resonator, and the pattern of changes in the oscillation frequency obtained from an adsorption-measuring type gas detector when organic gases of various kinds were brought into contact with the organic thin films was learned through a neural network model using back propagation, so as to discriminate the kind of gas. Meanwhile, the present inventors have proposed a novel optical gas detector in Published Unexamined Japanese Patent Application No. 243944/1990. This apparatus makes use of a phenomenon in which when an organic dye thin-film element is irradiated with excitation light, and an NO.sub.2 gas is brought into contact with the organic dye thin film, the intensity of fluorescence or phosphorescence emitted from the organic dye thin film changes. According to this apparatus, the fluorescence or phosphorescence emitted from the organic dye thin film changes noticeably owing to a slight electronic interaction between the dye molecules in the thin film and the gas molecules adsorbed on the thin film. Hence, by measuring the change in the fluorescence intensity or phosphorescence intensity, it is possible to detect a gas having a very low concentration. Furthermore, in a case where dye molecules are located adjacent to each other in the organic dye thin film and exist by forming a certain associated condition, its effect is amplified. If use is made of an organic dye thin film which assumes an associated condition formed by a very large number of molecules as in the case of J-aggregate(s), in particular, its amplification factor reaches several tens to hundreds fold.
In accordance with a conventional gas detecting method using a piezoelectric vibrating element, it is possible to ascertain an amount of a subject gas adsorbed on an organic thin film serving as a gas sensitive material. If a target gas is restricted, or the organic thin film on the piezoelectric vibrating element used has a particularly high distribution factor with respect to that gas, it is possible to measure the gas concentration quantitatively to a certain extent. In a case where the kind of gas is not restricted, or a coexisting gas is present, however, only limited information is obtained.
Meanwhile, an attempt has been made in which a plurality of arrangements in which an organic thin film is formed on such a piezoelectric vibrating element are prepared so as to discriminate the kind of gas on the basis of a difference in the amount of adsorption on the respective films. However, to distinguish between the difference of the gas concentration and the kind of gas only from the difference in the amount of adsorption, not only are required a multiplicity of gas detecting elements, but a burden imposed on the learning elements for effecting pattern recognition becomes large since the meaning of the signal patterns is difficult.
Meanwhile, in accordance with an optical gas detecting method making use of a change in fluorescence or phosphorescence, the kind of gas which imparts a change to the fluorescence or phosphorescence of the gas sensitive material can be detected with very high sensitivity. In addition, the difference in the property of the gas can be discriminated in the form of an increase or decrease in fluorescence or phosphorescence, and the gas concentration can be measured quantitatively from a rate of change thereof to a certain extent. With respect to the kind of gas which imparts no change to the optical characteristics, however, no signal is obtained.